1. Field of the Invention
The present invention relates to transparent quartz glass of high purity produced by molding powdery amorphous silica, and sintering and fusing the molded article of amorphous silica. Particularly, the present invention relates to pure transparent quartz glass which contains less impurities like alkalis respectively at a content of not more than 1 ppm, contains less water, and has high heat resistance, thereby being useful for a photomask and other uses in semiconductor production.
The present invention also relates to a process for producing the pure transparent quartz glass.
2. Description of the Related Art
Conventionally, transparent quartz glass is produced by a process of melting powdery quartz crystal in vacuo in a furnace, a process of melting powdery quartz crystal by oxyhydrogen flame, or a like process. However, the conventional processes employ powdered natural quartz crystal, whereby the resulting quartz glass is not satisfactory to meet the severe requirements for high purity for highly integrated LSI, even though the quartz glass has high heat resistance. Although the powdered natural quartz crystal is treated for high purity, not all of the undesired impurities can be removed to respective contents of not higher than 1 ppm at the moment.
To obtain higher purity of the quartz glass, processes are investigated, including a VAD process which sinters deposited synthetic amorphous silica, and a sol-gel process which sinters powdery amorphous silica obtained by hydrolysis of a silicic acid alkoxide. However, the synthetic powdery amorphous silica, although it can be produced in higher purity, gives quartz glass inferior in heat resistance to the one obtained from natural quartz material, disadvantageously. This disadvantage is presumed to be due to water included during the synthesis of the quartz glass, or the structure of the glass.
To offset the above disadvantage, a process is proposed for producing the glass by crystallizing amorphous silica and then fusing it. This process, which employs a crystallization promoter such as alkali or alumina, involves a problem of contamination with impurities which affect adversely the transparency to lower the quality of the product. The impurities cannot readily be eliminated. For example, elimination by heat treatment requires long time to lower the productivity.
Another process is proposed in which powdery amorphous silica is converted to powdery cristobalite and then it is fused. This process requires much time for grinding the powdery crystal, and resulting ground matter is poor in moldability disadvantagesouly, and is not suitable for industrial production.